Direct current (DC) architectures are well known, for example for the transmission and distribution of power. DC architectures generally provide efficient (low loss) distribution of electrical power relative to alternating current (AC) architectures.
The importance of DC architectures has increased because of factors including: (1) the reliance of computing and telecommunications equipment on DC input power; (2) the reliance of variable speed AC and DC drives on DC input power; and (3) the production of DC power by various renewable energy sources, such as photovoltaic solar panels.
The widespread use of DC architectures has also expanded the need for DC-DC power converter circuits. Moreover, there is a further need for DC-DC power converter circuits that are efficient and low cost.
Cost reduction is achieved in part by reducing the components of DC-DC power converters, for example by providing transformerless DC-DC converters. Two of the most common transformerless dc/dc converters are the buck converter 10, as shown in FIG. 1, for stepping down the voltage, and the boost converter 12, as shown in FIG. 2, for stepping up the voltage.
While both of these circuits are capable of achieving very high conversion efficiency when the input-to-output voltage ratio is near unity, their efficiency is less than optimal when the voltage ratio becomes high. Loss of efficiency, along with other operational problems, are caused by circuit parasitics, including such circuit effects as diode forward voltage drop, switch and diode conduction losses, switching losses, switch capacitances, inductor winding capacitance, and lead and trace inductances.
Furthermore, it is known in the prior art that boost converters in particular are susceptible to parasitic effects and high efficiency operation requires low step up ratios, e.g. 1:2 or 1:3. Higher step up ratios such as in the range of 1:10 or above are entirely impractical in light of cost and efficiency constraints, for example, as explained in N. Mohan, T. Undeland, W. Robbins, “Power electronics: converters, applications, and design,” Wiley, 1995.
B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, is an example of a resonant DC-DC power converter, where a resonant tank is excited at its resonant frequency to achieve high step-up/step-down conversion ratios without the use of transformers. An H-bridge based resonant DC-DC power converter was proposed by (D. Jovcic, “Step-up MW dc-dc converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407) and modified for enhanced modularity by A. Abbas and P. Lehn (A. Abbas, P. Lehn, “Power electronic circuits for high voltage dc to dc converters,” University of Toronto, Invention disclosure RIS#10001913, 2009-03-31).
There are a number of disadvantages to these prior art topologies.
The converter disclosed in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, requires two perfectly, or near to perfectly, matched inductors, each only utilized half of the time, to function properly. Perfect matching is not viable in many applications. Moreover, the fact that the inductor is only utilized half of the time effectively doubles the inductive requirements of the circuit. This is undesirable as the inductor is typically the single most expensive component in the power circuit. Furthermore, the converter in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, requires both a positive and negative input supply. This is often not available.
The converters disclosed in D. Jovcic, “Step-up MW dc-dc converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407, and A. Abbas, P. Lehn, “Power electronic circuits for high voltage dc to dc converters,” University of Toronto, Invention disclosure RIS#10001913, 2009-03-31, use four high voltage reverse blocking switching devices. For medium frequency applications (approx. 20 kHz-100 kHz) such devices are not readily available thus they need to be created out of a series combination of an insulated-gate bipolar transistor (“IGBT”) and a diode, or a MOSFET and a diode. This not only further increases system cost but it also nearly doubles the device conduction losses of the converter.